Hardening Off Plants: Common Reader Questions and Answers

by Erin Marissa Russell

Wondering what cold acclimation is all about, and when, why, and how to follow the hardening off process? As a gardener, you don’t want to see your precious plants die shortly after they arrive from an online order or once they’ve grown from seeds into young plants  just because they needed hardening off. But never fear because we’re here to explain when, why, and how to follow the cold acclimation process.

This article will arm you with the knowledge to determine which plants need to go through cold acclimation, explain what cold acclimation is and why it’s needed, and give you step-by-step instructions for hardening off the plants that benefit from gradual introduction to an outdoor garden.

How do you know whether plants need cold acclimation or hardening off?

Cold acclimation, also called “hardening off,” lets plants get used to cold weather a little at a time so it won’t shock them, which can cause damage or even death. You’ll need to acclimate plants that are shipped to you because they may come from a location with warmer weather or they may have been raised in a greenhouse. Plants with tender young leaves or new growth will also need to be acclimated before introducing them to chilly temperatures. (And hardening off isn’t just a good idea in cold weather—your new plants may need to be acclimated to get used to direct sunlight, for example.)

How do you harden off plants?

Before you get new plants situated in their permanent homes, there are a few steps to follow to acclimate them to cold weather (or direct sunlight). First, leave plants in the container they came in and find a shady spot outside for them that provides some shelter from the wind. A porch or area between outdoor buildings may work, or you can use a patio that isn’t climate controlled.

Let your plants get used to the weather in this location for three to four hours the first day, then bring them back indoors. The next day, extend their time in the sheltered, shady spot for one or two hours. Continue adding an hour or two each day, bringing your plants indoors after their time outside, for three or four days after their arrival.

After the first three or four days in their sheltered spot, you’ll need to switch to a location where plants will get some morning sunshine. Once afternoon rolls around, return your new plants to the sheltered spot you first selected. At night, bring plants indoors once again.

Keep this up until your plants have had seven to 10 days to get used to the cold weather. (If your schedule or another conflict makes it impossible to give your plants morning sun, you can choose a spot that provides filtered sunlight instead. Just be sure that no matter what, your plants make it indoors at night, when the weather is coldest.)

Remember to water your plants according to their care requirements throughout the hardening off process. Also, keep a watchful eye on the leaves of your plants to check for signs of damage from the sunlight or chilly weather. If you see signs of shock or sunscald, go back to the first step to give the plants some extra time to ease into the great outdoors.

Once you’ve followed these steps, your plants are ready to be moved into their permanent home in your outdoor garden, as long as temperatures will stay above 50 degrees Fahrenheit while they get adjusted. If a cold snap is expected, just keep going with the hardening off process until the weather warms back up. Planting on a cloudy day is best, if possible, for the easiest transition into your garden.

What happens if you don’t harden off plants?

If you don’t harden off your new arrivals and it turns out they needed cold acclimation, the consequences can be dire. Plants exposed to direct sunlight before they’re ready can suffer from sunscald, which burns the leaves and can shock a plant (especially one that’s already in shock from shipping or being moved). In a worst-case scenario, plants that needed hardening off and didn’t get it can simply die.

When a plant is exposed to weather that’s too cold for it, freezing damage comes into play. The water that makes up plant cells can freeze inside the plant, dehydrating them and wreaking havoc on the cell walls. Then when the weather warms and plants defrost, the sudden change results in cold injury, killing the affected stems and leaves. Even trees, which we tend to think of as hardy to cold weather, can suffer from cold injury if they’re young or have thin bark. That’s why it’s better safe than sorry when you’re dealing with new arrivals or young plants in your garden.

If you’re not sure whether a plant needs to be acclimated, it’s a good rule of thumb to put them through the hardening off process. After all, it can’t hurt, and following the cold acclimation process could save the cost and heartache of losing a new plant. After dreaming about a plant on your wish list, shelling out for young plants or babying them from seeds, and counting the days until they arrive or mature, it would be a shame to lose them to shock when a few simple steps could keep them healthy and thriving.

Want to learn more about cold acclimation?

Albopepper covers Harden Off Seedlings

The Charlotte Observer covers Keep Plants from Freezing

Gardening Know How covers Tips for Saving Cold Damaged Plants

Stark Bro’s Nurseries & Orchards Co. covers Acclimating New Plants & Trees